The 'sequential focus navigation starting point' feature defines where we start to search for focusable elements for sequential focus navigation ([Tab] or [Shift-Tab]) when there is no focused area. It's especially helpful for accessibility features like skip links and managing focus in the document.

CSS variables, more accurately known as CSS custom properties, are landing in Chrome 49. They can be useful for reducing repetition in CSS, and also for powerful runtime effects like theme switching and potentially extending/polyfilling future CSS features.

In this codelab, you'll learn how to debug a service worker using the new DevTools Application panel. You'll also learn how to simulate a Push notification to verify your subscription is properly setup.